Clinical Note

A common medical condition is hemorrhage (blood loss). The resulting hypovolemia moves the equilibrium point down and to the left with a decreased cardiac output and venous pressure. Reduced cardiac output by this mechanism is the primary problem facing these patients and explains why transfusion is the only rational treatment for the hemorrhagic patient. If blood volume is not promptly restored, the peripheral tissues will be irreversibly injured because of inadequate blood flow, and hemorrhagic shock will ensue. A low blood pressure is usually the most obvious sign of hypovolemia and the physician may be tempted to try to increase it with an arteriolar constrictor like phenylephrine. Although the constrictor would raise arterial pressure, it would also further depress cardiac output and only make the situation worse.

Intrathoracic Pressure Affects the Cardiac Function Curve

Intrathoracic pressure is the pressure presented to the outside of the heart. The pressure surrounding the heart acts in parallel with the compliance of the cardiac musculature to oppose ventricular distension by the venous pressure. End-diastolic volume will, therefore, be determined by the difference between venous pressure and intrathoracic pressure. As a result, changing intrathoracic pressure shifts the cardiac function curve horizontally along the x axis. Normally, intrathoracic pressure is 2-3 mm Hg negative because of the breathing mechanics. Increasing the intrathoracic pressure to atmospheric by opening the chest will reduce the cardiac output and increase the venous pressure because it will shift the ventricular output curve about 2-3 mm to the right.

actively pump blood into the veins feeding the failing side, raising venous pressure on that side to 20-30 mm Hg. That is enough pressure to force fluid out of the capillaries and cause edema upstream of the failing ventricle.

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